D600 RAW is not much better than A900

If we simplify things a bit (or a lot...), imagine the electronics being a fixed size (as small as reasonably possible), thus blocking a fixed sized portion of the light reaching a photodiode. The smaller the photodiode, the larger the relative portion of light being blocked, the more loss of photons per fixed unit area.

Again, greatly simplified, but that's largely the reason why small pixel backlit sensors gain back much more light per fixed unit area than large pixel sensors. For 1/2.3" 12 MP sensors that can mean up to a stop diference in light or noise. Keeping those tiny electronics constant, that's much less than 1/6th of a stop for current FF sensors. Which doesn't warrant the extra costs at this point. By the time we see well over 100MP FF sensors, you'll probably see this technique making its way into such large sensors as well, because the pixels will be small enough to benefit visibly from it.